THE Chook had a few queries from readers after last week’s story about Fremantle’s new traffic bridge about what exactly made a bridge “extradosed”

Extradosed bridges, a relatively recent innovation in bridge design, emerged in the late 20th century as a hybrid solution that blends features of cable-stayed and conventional girder bridges. 

The term “extradosed” is derived from Latin, meaning “above the arch,” and refers to the location of the bridge’s cables, which are situated above the bridge deck but below the main girder. 

• How Main Roads says Fremantle’s new traffic bridge might look.

This distinctive configuration sets extradosed bridges apart, offering a unique combination of aesthetic appeal and structural efficiency.

While several bridges incorporated elements of these design principles in the 1980s, the Ponte dos Socorridis in Portugal (1993) is credited with being the first after engineer Jacques Mathivat codified the concepts. 

The success of this and several Japanese projects spurred interest and exploration into extradosed bridges as an alternative to traditional cable-stayed and suspension bridges. 

Engineers sought a solution that could span medium to long distances while mitigating the visual impact and cost associated with extensive cable arrangements.

The design of extradosed bridges addresses these concerns by distributing the load more efficiently compared to traditional cable-stayed bridges. 

The cables, positioned between the tower and the deck, reduce the need for tall towers and extensive cable systems, resulting in a more streamlined and visually pleasing structure. This unique configuration also enhances the bridge’s resistance to dynamic loads and vibrations.

Extradosed bridges have since gained popularity worldwide for their adaptability to various spans and topographical conditions and there are now around 50, with most in Japan and South Korea.